An unconfounded Scheffe Mix approach was used to probe important ions and their interactions in supporting biomass and lipid production by Chlorella sp. EN1234. Six major cations and anions; NH4+, NO3-, Na+, K+ PO4 and Cl- were examined. Piepel plots and RSM analysis showed that in a number of cases, the major media anions PO4- and Cl- negatively influence final cell densities, and that maximal cell density is obtained with nitrate over ammonium, with an optimal effect when mixed with equal molar potassium. As well, although it is commonly assumed that lipid content increases in nitrogen deficient media, here little correlation between nitrogen content and total lipid content was found with mixtures that supported high lipid productivity. Thus these mixtures define the composition space within which further R&D might produce the best trade-off between total biomass production and high cellular lipid content. (C) 2014 Elsevier Ltd. All rights reserved.